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   "source": [
    "This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Challenge Notebook"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Problem: Implement a stack with push, pop, peek, and is_empty methods using a linked list.\n",
    "\n",
    "* [Constraints](#Constraints)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Unit Test](#Unit-Test)\n",
    "* [Pythonic-Code](#Pythonic-Code)\n",
    "* [Solution Notebook](#Solution-Notebook)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Constraints\n",
    "\n",
    "* If we pop on an empty stack, do we return None?\n",
    "    * Yes\n",
    "* Can we assume this fits memory?\n",
    "    * Yes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "### Push\n",
    "\n",
    "* Push to empty stack\n",
    "* Push to non-empty stack\n",
    "\n",
    "### Pop\n",
    "\n",
    "* Pop on empty stack\n",
    "* Pop on single element stack\n",
    "* Pop on multiple element stack\n",
    "\n",
    "### Peek\n",
    "\n",
    "* Peek on empty stack\n",
    "* Peek on one or more element stack\n",
    "\n",
    "### Is Empty\n",
    "\n",
    "* Is empty on empty stack\n",
    "* Is empty on one or more element stack"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "Refer to the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/stacks_queues/stack/stack_solution.ipynb).  If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Node(object):\n",
    "\n",
    "    def __init__(self, data):\n",
    "        # TODO: Implement me\n",
    "        pass\n",
    "\n",
    "\n",
    "class Stack(object):\n",
    "\n",
    "    def __init__(self, top=None):\n",
    "        # TODO: Implement me\n",
    "        pass\n",
    "\n",
    "    def push(self, data):\n",
    "        # TODO: Implement me\n",
    "        pass\n",
    "\n",
    "    def pop(self):\n",
    "        # TODO: Implement me\n",
    "        pass\n",
    "\n",
    "    def peek(self):\n",
    "        # TODO: Implement me\n",
    "        pass\n",
    "\n",
    "    def is_empty(self):\n",
    "        # TODO: Implement me\n",
    "        pass"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test\n",
    "\n",
    "\n",
    "\n",
    "**The following unit test is expected to fail until you solve the challenge.**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# %load test_stack.py\n",
    "import unittest\n",
    "\n",
    "\n",
    "class TestStack(unittest.TestCase):\n",
    "\n",
    "    # TODO: It would be better if we had unit tests for each\n",
    "    # method in addition to the following end-to-end test\n",
    "    def test_end_to_end(self):\n",
    "        print('Test: Empty stack')\n",
    "        stack = Stack()\n",
    "        self.assertEqual(stack.peek(), None)\n",
    "        self.assertEqual(stack.pop(), None)\n",
    "\n",
    "        print('Test: One element')\n",
    "        top = Node(5)\n",
    "        stack = Stack(top)\n",
    "        self.assertEqual(stack.pop(), 5)\n",
    "        self.assertEqual(stack.peek(), None)\n",
    "\n",
    "        print('Test: More than one element')\n",
    "        stack = Stack()\n",
    "        stack.push(1)\n",
    "        stack.push(2)\n",
    "        stack.push(3)\n",
    "        self.assertEqual(stack.pop(), 3)\n",
    "        self.assertEqual(stack.peek(), 2)\n",
    "        self.assertEqual(stack.pop(), 2)\n",
    "        self.assertEqual(stack.peek(), 1)\n",
    "        self.assertEqual(stack.is_empty(), False)\n",
    "        self.assertEqual(stack.pop(), 1)\n",
    "        self.assertEqual(stack.peek(), None)\n",
    "        self.assertEqual(stack.is_empty(), True)\n",
    "\n",
    "        print('Success: test_end_to_end')\n",
    "\n",
    "\n",
    "def main():\n",
    "    test = TestStack()\n",
    "    test.test_end_to_end()\n",
    "\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    main()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Solution Notebook\n",
    "\n",
    "Review the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/stacks_queues/stack/stack_solution.ipynb) for a discussion on algorithms and code solutions."
   ]
  }
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